The proteasome is a massive multi-catalytic protease complex that is responsible for degrading the majority of cellular proteins. The 20S-core particle of the 26S proteasome is barrel-shaped, and the sites of proteolytic activity reside on the interior.
The eukaryotic proteasome contains three known activities that are associated with its β subunits. These are the chymotrypsin-like (cleavage after hydrophobic residues, β5 subunit), trypsin-like (cleavage after basic residues, β2 subunit), and caspase-like (cleavage after acidic residues, β1 subunit) activities.
These three activities depend on the presence of an N-terminal Thr (Thr 1) residue. The hydroxyl group on the side chain of Thr 1 is responsible for catalyzing cleavage of peptides through nucleophilic attack (addition-elimination mechanism). Near this N-terminal threonine, binding pockets recognize the side chains of peptides and give each catalytic site its specificity. The S1 pocket of the β5 subunit is defined by the hydrophobic residues, Ala 20, Val 31, Ile 35, Met 45, Ala 49, and Glu 53, and this binding pocket has been shown to be important for substrate specificity and binding of several types of proteasome inhibitors.
The ubiquitin/proteasome-dependent degradation pathway plays an essential role in upregulation of cell proliferation, down-regulation of cell death, and development of drug resistance in human tumor cells, suggesting the use of proteasome inhibitors as potential novel anticancer drugs, which has been demonstrated in various cell cultures, animal models and clinical trials. In a broad range of cell culture models, proteasome inhibitors rapidly induce tumor cell apoptosis, selectively trigger programmed cell death in the oncogene-transformed, but not normal or untransformed cells, and are able to activate the death program in human cancer cells that are resistant to various anticancer agents. Inhibition of the chymotrypsin-like, but not trypsin-like, activity has been found to be associated with induction of tumor cell apoptosis.
The proteasome degrades a number of proteins that are involved in tumor suppression. Cyclin-dependent kinase inhibitor p27, a key regulatory molecule in cell cycle progression, is one example (Pagano, M. et al. Science, 1995, 269:682-685). Inhibition of the proteasome results in an accumulation of ubiquitinated and unmodified p27′ that can result in G1 cell cycle arrest (An, B. et al. Cell Death Differ, 1998, 5:1062-75; Sun, J. et al. Cancer Res, 2001, 61:1280-1284). Additionally, inhibition of the proteasome increases the intracellular concentrations of IκB-a (Palombella, V. J. et al. Cell, 1994, 78:773-785), an inhibitor of nuclear factor kappa B (NFκB), leading to inhibition of NFκB activation (Thompson, J. E. et al. Cell, 1995, 80:573-582) and reduction of anti-apoptotic gene signaling (Perkins, N. D. Trends Biochem Sci, 2000, 25:434-440). Another effect of proteasome inhibition is the accumulation of mitochondrial proapoptotic protein Bax, a Bcl-2 family member (Chang, Y. C. et al. Cell Growth Differ, 1998, 9:79-84; Li, B. and Dou, Q. P. Proc Natl Acad Sci USA, 2000, 97:3850-3855; Nam, S. et al. Cancer Epidemiol Biomarkers Prey, 2001, 10:1083-1088), resulting in the release of cytochrome c from the mitochondria and activation of caspase-mediated apoptosis (Green, D. R. and Reed, J. C. Science, 1998, 281:1309-1312).
In different animal studies, proteasome inhibitors suppress tumor growth via induction of apoptosis and inhibition of angiogenesis. MLN-341 (formerly PS-341) is a potent and selective dipeptidyl boronic acid compound, which inhibits the chymotrypsin-like activity of the 20S proteasome. This proteasome inhibitor is currently being developed for the potential treatment of human hematological malignant neoplasms and solid tumors. Preliminary data from Phase I and II clinical trials confirm the anti-tumor activity of MLN-341 although some associated side effects were observed. The proteasome inhibition mechanism of MLN-341 has not been confirmed by X-ray diffraction experiments.
However, the proteasome-inhibition mechanism of another peptide inhibitor, LLnL, and nonpeptide inhibitors, such as lactacystin and the macrocyclic compound TMC-95, have been confirmed by X-ray diffraction. Understanding how these inhibitors function at the molecular level will give insight into the structural studies of other proteasome inhibitors where X-ray crystal structures are not available. These studies thereby demonstrate that the proteasome is an excellent target for developing pharmacological anti-cancer drugs.
Tea, the most popular beverage in the world, is consumed by over two-thirds of the world's population. Several epidemiological studies have provided evidence for the cancer-preventive properties of green tea. Furthermore, animal studies have also suggested that green tea polyphenols could suppress the formation and growth of various tumors. Although numerous cancer-related proteins are affected by tea polyphenols, the molecular basis for tea-mediated cancer prevention remains unknown.
The naturally occurring ester bond-containing green tea polyphenols (GTPs), such as (−)-epigallocatechin-3-gallate (also referred to herein as (−)-EGCG, and shown in FIG. 1), possess the ability to inhibit proteasome activity both in vitro and in vivo. Recently completed Phase I clinical trials using (−)-EGCG and green tea to treat cancer and prevent reoccurrence indicate a wide tolerance to green tea (up to 7-8 cups/per day) (Pisters, K. M. et al. J Clin Oncol, 2001, 19:1830-1838). The lack of toxicity to normal cells observed in clinical trials and effectiveness of treatment confirm the results from the cell culture models (Adams, J. et al. Cancer Res, 1999, 59:2615-2622; Dou, Q. P. and Li, B. et al. Drug Resist Updat, 1999, 2:215-223; Almond, J. B. and Cohen, G. M. Leukemia, 2002, 16:433-443; Kisselev, A. F. and Goldberg, A. L. Chem Biol, 2001, 8:739-758). In addition, synthetic GTPs with an ester bond, such as (+)-EGCG (shown in FIG. 1), are also able to potently and selectively inhibit the chymotrypsin-like activity of the proteasome. It appears that a center of nucleophilic susceptibility resides at the ester bond carbon in these polyphenols. This proposed mechanism of ester bond-based nucleophilic attack is similar to that of lactacystin-based inhibition. However, a need still exists for more options in inhibiting proteasome activity.